import java.awt.*;
// export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/fortran/lib:/usr/local/fortran/lib64
// /usr/local/fortran/bin/gcj --main=RungeKutta RungeKutta.java -o RK
 
public class RungeKutta extends Frame  implements Runnable {
    int x,y;         //window coordinates of the mass
    double h=0.02;   // time increment
    double q=1.0;    //initial data
    double p=0.0;
    double k1,k2,l1,l2,k3,l3; // RK variables
    int frameNumber = -1;
    Thread RKThread;
    boolean stopped = false;

    RungeKutta(int fps, String windowTitle) {
        super(windowTitle);
    }

    public void startRK() {
        if (stopped) {
        } else {
            if (RKThread == null) {
                RKThread = new Thread(this);
            }
            RKThread.start();
        }
    }

    public void stopRK() {
        RKThread = null;
    }

    public boolean mouseDown(Event e, int x, int y) {
        if (stopped) {
            stopped = false;
            startRK();
        } else {
            stopped = true;
            stopRK();
        }
        return true;
    }

    public void run() {
        Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
        Thread currentThread = Thread.currentThread();
        //All of the work is done here
        while (currentThread == RKThread) {
            frameNumber++;
	    double t=h*(double)frameNumber;	    
	    k1=h*p;
	    l1=-2.0*h*q*q*q;
	    k2=h*(p+0.5*l1);
	    l2=-2.0*h*(q+0.5*k1)*(q+0.5*k1)*(q+0.5*k1);
	    k3=h*(p+l2);
	    l3=-2.0*h*(q+k2)*(q+k2)*(q+k2);
	    q=q+(k1+4.0*k2+k3)/6.0;
	    p=p+(l1+4.0*l2+l3)/6.0;
	    x=(int)(40.0*q);
	    y=(int)(40.0*p);	    
	    repaint();
	    try {Thread.sleep(5); }
	    catch (InterruptedException e){break;}
	}    
    }

    public void paint(Graphics g) {    
	g.setColor(Color.white);
	g.drawRect(0,0,250,250);
	g.setColor(Color.black);
	g.drawLine(125, 30, 125, 220);
	g.drawLine(30, 125, 220, 125);
	g.drawString("q",210, 135);
	g.drawString("p",130, 40);
	g.setColor(Color.red);
        g.drawOval(125+x,125+y,4,4);
    }

    public static void main(String args[]) {
        RungeKutta RK = null;
        int fps = 60;     // lots of frames per second   
        RK = new RungeKutta(fps, "Oscillator");
        RK.setSize(250, 250);
        RK.show();
        RK.startRK();
    }
}